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Weathering and Erosion Weathering – the disintegration and decomposition of material at the surface Erosion – the transportation of weathered material by water, wind, or ice Weathering Two kinds of weathering • Mechanical weathering • Chemical weathering Weathering along joints – Balanced Rock, Joshua Tree National Monument Weathering along joints – Balanced Rock, Joshua Tree National Monument Unloading and exfoliation of igneous rocks Exfoliation joints – Tioga Road, Yosemite National Park Exfoliation joints – Yosemite National Park: Half Dome Ice wedging – expansion of water due to freezing Salt weathering – growth of salt crystals due to evaporation of water Prying apart of rocks due to growth of tree roots along joints and fractures Water transport Wind transport Ice transport Weathering • Chemical weathering • Dissolution of minerals • Chemical alteration of minerals by removal/addition of elements Acids: chemical solutions in which hydrogen ions (H+) are released Formation of carbonic acid CO2 + H2O water and carbon dioxide Formation of carbonic acid CO2 + H2O water and carbon dioxide H2CO3 carbonic acid Formation of carbonic acid CO2 + H2O water and carbon dioxide H2CO3 carbonic acid H+ + HCO3- hydrogen ion + bicarbonate ion Formation of carbonic acid CO2 + H2O water and carbon dioxide H2CO3 carbonic acid H+ + HCO3- hydrogen ion + bicarbonate ion 2H+ + CO32- 2 hydrogen ions + carbonate ion Carbonic acid in the ocean Chemical weathering -- dissolution of calcite Granite Limestone Chemical weathering – dissolution of limestone or marble Weathering Chemical weathering of silicate minerals • Goldich’s stability series • Stability of silicate minerals increases with the percentage of Si-O bonds: • Quartz – all Si-O bonds, most stable • Feldspars • Micas • Hornblende • Augite • Olivine – fewest Si-O bonds, least stable Weathering Chemical weathering of silicate minerals • Weathering of mafic minerals produces insoluble iron oxides and clay minerals Weathering Chemical weathering of silicate minerals • Weathering of mafic minerals produces insoluble iron oxides and clay minerals • Weathering of feldspars produces clay minerals and silica in solution Weathering Chemical weathering of silicate minerals • Weathering of mafic minerals produces insoluble iron oxides and clay minerals • Weathering of feldspars produces clay minerals and silica in solution • Quartz remains substantially unaltered Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 orthoclase Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 + 2H2O orthoclase water Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 + 2H2O + 2H+ orthoclase water hydrogen ions Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 + 2H2O + 2H+ orthoclase water hydrogen ions 2K+ potassium ions Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 + 2H2O + 2H+ orthoclase water hydrogen ions Al2Si2O5(OH) + 2K+ kaolinite potassium ions Formation of Kaolinite, a clay mineral, from the weathering of Orthoclase 2KAlSi3O8 + 2H2O + 2H+ orthoclase water hydrogen ions Al2Si2O5(OH) + 2K++ 2SiO20 kaolinite potassium ions silica in solution Clay crystals have a charge separation, with positive charges sandwiched between layers of negative charge, making them attractive to water molecules 10 microns = 0.01 mm Spheroidal weathering – chemical weathering along surfaces and joint planes Sandcastles collapse when sand dries (pores contain only air) or when sand is saturated (pores contain only water) Sand grains stick together while damp, due to surface tension -- when pores contain water and air, water molecules stick to each other and to sand grains When dry, clay holds about 20% water -- sculptures thus hold their shape through the firing process Rates of weathering Advanced mechanical weathering aids chemical weathering by increasing the surface area Important factors • Rock characteristics • Mineral composition and solubility • Physical features such as joints Rates of weathering Important factors • Climate • Temperature and moisture are the most crucial factors • Chemical weathering is most effective in areas of warm temperatures and abundant moisture
